blktrace: Create empty output files for non-existent cpus

blktrace: Create empty output files for non-existent cpus

When CPU number space is sparse, we don't start threads for non-existent
CPUs. As a result, there are no output files created for these CPUs
which confuses tools like blkparse which expect that CPU numbers are
contiguous. Create fake empty files for non-existent CPUs so that other
tools don't have to bother.

Note that in network mode, the server will create all files in the range
0..max_cpus automatically.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>